Valves which control the flow of hazardous or valuable media are often provided with a latching device as a safety feature to prevent inadvertent operation; such safety devices require a deliberate preparatory action (releasing the latch) to render the valve operable. In the typical example, the latch automatically reengages when an operation has been completed.
While these prior art latching devices serve their intended purpose when operators are accustomed to their use or when the operating technique is self-evident, circumstances exist under which these latching devices become safety hazards themselves.
It is not unusual to find latching valves in applications where timely operation is crucial in preventing or remedying a hazard, a cargo valve which controls the flow of hazardous media provides an example. The cargo valve is opened in normal course to dispense cargo, then closed when delivery is completed. Safety considerations suggest that such a valve be secured in the closed position by a latching device requiring a deliberate action (unlatching) before being operable. However, since timely closing of this valve may be critical in avoiding a spill, the presence of a latching device becomes an impediment.
In other applications, a dangerous condition is created by closing, rather than opening, a valve. Manual valves used in emergency pressure relief applications must be readily operable in the closed-to-open direction then secured in the open position until the underlying cause of the over pressure problem is addressed. Encumbering such a valve with a latch that latches in both the open and closed positions compromises its function as an emergency device.
In many installations, valves are partially concealed; tank drain valves, unloading valves and valves used as components of equipment built into cabinets are often hidden from view. An individual intent on operating a latching valve not in full view must first be familiar with the latching device, or be willing to spend some time fumbling. In an emergency, an individual who is imperilled may pause in his flight to give a valve handle a quick twist, but he will almost certainly be unwilling to spend precious time contending with a balky latch whose operation he does not understand.